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Sunday, 24 July 2011

Drunken forests and cable trays

As we discussed in the last article, cables for LOFAR stations are typically buried. This is not possible at KAIRA. The reason for this is the problems caused by the freezing conditions. The fact that the soil will soak with water and then freeze means that trapped cables can be stretched, crimped or even broken under the strain of the ice. Because of the volume of snow (and hence melt-water) and the depth of the ground freezing, this is a serious problem.

KAIRA is the only deployment of LOFAR technology in a boreal subarctic climate. This sort of environment is subject to frost heaving, ice-lensing, palsas and hydrolaccoliths. These are the sorts of phenomena that lead to a arboreal effect which can be seen in the region. This is where ground distortion can cause trees to lean at a seemingly random angles because of soil displacement around the root base. Trees affected by such ground upheaval are said to be drunk and the large areas where this occurs are referred to as 'juopunut metsä' (= drunken forest).

Although one or two seasons might be okay, the lifetime of the KAIRA project is a minimum of 15 years and over these sorts of timescales the effects can be significant - especially in the case of an abnormal precipitation or freezing event.

We anticipate that the site will be affected by this and so distortions of the power poles, reindeer fence and other such installations is to be expected. This is partly the reason for the raised platforms for all the KAIRA antennas, as it will allow us to adapt the framesets to ground distortion, or to get under the array, jack up the affected locations and re-level the soil.

However, also as a result of the ground movement, the conventional burying of the signal cables is not a option. Instead we will route our cables on timber trays above ground, just under the antenna tiles themselves. Because of gaps in the array, there will be the need to dip down temporarily underground, but this needs to be done through flexible ducting, to ensure that ground movement will not lock into the cables and affect them — that is, the cables must have free movement in these tunnels. Of course, it might be conceivable to route all the cables in free-air ducts underground, but this is prohibitive in our case because of time, cost and access issues. So, cable trays it is, and these are now under production.

Mikko Tilja and Pertti Nissinen during the preparation of a feeder cable tray.

The first trays themselves are a 'V' shape, made from two long planks nailed together. These are stock-piled for later deployment under the array.

Some completed 'V' trays.

The installation of the cable trays is a pretty major task and has taken up the bulk of the time between the completion of the antenna tile installation and the delivery of the electronics and cabling.